Poxviruses are double-stranded DNA viruses whose protein core is surrounded by a lipid envelope. Embedded into the lipid envelope are four conserved viral proteins which are thought to mediate viral binding. While the function of these proteins has been studied in vitro, their impact on poxviral tropism and pathogenesis in vivo remains unknown. We now present data demonstrating that removal of one of these envelope proteins, the putative chondroitin binding protein M083L, from the leporipoxvirus myxoma significantly attenuates viral pathogenesis in an evolutionarily relevant host species by limiting systemic viral dissemination. Critically, loss of M083 severely restricts myxoma virus tropism for activated T cells providing a direct mechanism through which this protein impacts viral dissemination. The roles of the remaining three poxviral envelope proteins, however, remains unknown. We therefore put forth the current proposal designed to identify how each of the conserved poxviral envelope proteins impacts viral tropism and disease pathogenesis by: removing each viral envelope protein from the genome of MYXV (Specific Aim 1), characterizing how removal of these envelope proteins impacts viral replication in vitro (Specific Aim 2), and identifying how removal of each viral envelope protein impacts viral tropism and pathogenesis in vivo (Specific Aim 3). We anticipate that the proposed studies will uncover how each individual poxviral envelope protein impacts viral pathogenesis and tropism in vivo. These studies will not only advance our understanding of basic poxviral biology but will also significantly impact public health both by allowing for rationale design of poxviral vaccination strategies and improving poxvirus-based oncolytics by increasing our understanding of how to alter viral tropism in vivo to increase viral dissemination and delivery to sites of residual disease.